In recent years, rechargeable electric storage devices such as battery cells (lithium ion battery cells, nickel hydrogen battery cells, etc.) and capacitors (electric double layer capacitors, etc.) are employed as a power source, for example, for vehicles (automobiles, motorcycles, etc.) and various equipment (mobile terminals, laptop computers, etc.).
For example, a battery cell (can-type battery cell) disclosed in Patent Literature 1 includes: an electrode assembly (power generating element); and a case (housing case) having a case body (can) that houses the electrode assembly and a cover plate (cover) that closes an opening of the case body. In a method for producing such a case body, an unnecessary portion such as a burr remains at an opening edge of the opening of the case body. This unnecessary portion is subjected to machining such as trimming, so as to be cut off from the opening edge of the opening of the case body. Therefore, a step portion that is one step lower than other portions of the opening edge of the opening that have not undergone machining may be formed in some cases at the opening edge of the opening where the unnecessary portion has been present.
In such a case, this step portion forms a gap between the cover plate and the opening edge of the opening of the case body, when the cover plate is placed on the opening of the case body. Eventually, when a seam between the cover plate and the opening edge of the opening of the case body is laser welded, the laser beam penetrates through the gap into the case. Thereby, the laser beam may possibly affect the contents such as a resin plate provided within the case.
The step portion formed at the opening edge of the opening of the case body also includes a step portion formed in a portion of the opening for optically detecting the orientation of the case, in addition to the above-mentioned step portion. Such a step portion also has a possibility of affecting the contents.
A battery cell (rectangular battery cell) disclosed in Patent Literature 2 includes an electrode assembly (battery element), a case body (body case) that houses the electrode assembly, and a cover plate (cover) that closes an opening of the case body. The cover plate is provided with a base portion projecting toward the inside of the case body. This battery case is sealed by fitting the base portion of the cover plate into the opening of the case body.
Therefore, when the seam between the cover plate and the opening edge of the opening of the case body is laser welded, even if the laser beam has passed through this gap, the base portion of the cover plate blocks the way of the laser beam. Therefore, the laser beam does not penetrate into the contents within the case.
However, the base portion of Patent Literature 2 planarly projects uniformly from the cover plate toward the inside of the case body. Therefore, a capacity equivalent to the volume of the base portion is not available as a capacity to house the electrode assembly within the case. That is, the capacity equivalent to the volume of the base portion of the cover plate is lost from the capacity of the case. Since the capacity equivalent to the volume of the base portion of the cover plate does not contribute to the electricity storage by the battery cell, the electric storage capacity per unit volume of the battery cell is reduced.
Such problems are not limited to batteries, and the same problems apply also to capacitors (such as electric double layer capacitors).